ipv6: Return error for RTA_VIA attribute

IPv6 currently does not support nexthops outside of the AF_INET6 family.
Specifically, it does not handle RTA_VIA attribute. If it is passed
in a route add request, the actual route added only uses the device
which is clearly not what the user intended:

  $ ip -6 ro add 2001:db8:2::/64 via inet 172.16.1.1 dev eth0
  $ ip ro ls
  ...
  2001:db8:2::/64 dev eth0 metric 1024 pref medium

Catch this and fail the route add:
  $ ip -6 ro add 2001:db8:2::/64 via inet 172.16.1.1 dev eth0
  Error: IPv6 does not support RTA_VIA attribute.

Fixes: 03c0566 ("mpls: Netlink commands to add, remove, and dump routes")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -4182,6 +4182,10 @@ static int rtm_to_fib6_config(struct sk_buff *skb, struct nlmsghdr *nlh,
 		cfg->fc_gateway = nla_get_in6_addr(tb[RTA_GATEWAY]);
 		cfg->fc_flags |= RTF_GATEWAY;
 	}
+	if (tb[RTA_VIA]) {
+		NL_SET_ERR_MSG(extack, "IPv6 does not support RTA_VIA attribute");
+		goto errout;
+	}
 
 	if (tb[RTA_DST]) {
 		int plen = (rtm->rtm_dst_len + 7) >> 3;